In an electrostatographic copy machine an electrostatic latent image is formed on an element. That image is developed by the application of an oppositely charged toner to the element. The image-forming toner on the element is then transferred to a receiver where it is fixed. The transfer of the toner to the receiver is usually accomplished electrostatically, by means of an electrostatic bias between the receiver and the element.
In order to produce copies of very high resolution, it is necessary to use toner particles that have a very small particle size, i.e., less than about 8 micrometers. (Particle size herein refers to mean volume weighted diameter as measured by conventional diameter measuring devices such as a Coulter Multisizer, sold by Coulter, Inc. Mean volume weighted diameter is the sum of the mass of each particle times the diameter of a spherical particle of equal mass and density, divided by total particle mass.) However, it has been found that it is very difficult to electrostatically transfer such fine toners from the element to &he receiver, especially when they are less than 6 micrometers in diameter. That is, fine toner particles frequently do not transfer from the element with reasonable efficiency. Moreover, those particles which do transfer frequently fail to transfer to a position on the receiver that is directly opposite their position on the element, but rather, under the influence of coulombic forces, tend to scatter, thus lowering the resolution of the transferred image and increasing the grain and mottle.
In order to avoid this problem, it has become necessary to transfer the toner from the element to the receiver by non electrostatic processes. One such process is the thermally assisted transfer process where the receiver is heated, typically to about 60.degree. to about 90.degree. C., and is pressed against the toner particles on the element. The heated receiver sinters the toner particles, causing them to stick to each other and to the receiver, thereby effecting the transfer of the toner from the element to the receiver. The element and receiver are then separated and the toner image is fixed, e.g., thermally fused to the receiver. For details, see copending application Ser. No. 230,394, titled "Thermally Assisted Transfer of Small Electrostatographic Toner Particles" filed Aug. 9, 1988, and herein incorporated by reference.
While the thermally assisted transfer process does transfer very small particles without the scattering that occurs with electrostatic transfer processes, it may sometimes be difficult to transfer all of the toner particles by this process. The toner particles that are directly on the element often experience a greater attractive force to the element than they do to the receiver and to other toner particles that are stacked above them, and the heat from the receiver may have diminished to such an extent by the time it reaches the toner particles next to the element that it does not sinter them. As a result, the toner particles that are in contact with the element may not transfer. Attempts to solve this problem by coating the element with a release agent have not proved to be successful because the process tends to wipe the release agent off the element into the developer, which degrades both the developer and the development process.
An alternative approach to removing all of the toner particles from the element is to use a receiver that has been coated with a thermoplastic polymer. During transfer the toner particles adhere to or become partially embedded in the thermoplastic coating and are thereby removed from the element. However, it has been found that many thermoplastics that are capable of removing all of the toner particles also tend to adhere to the element. This, of course, not only seriously impairs image quality but it may also damage both the element and the receiver. Until now no good solution has been found to this problem.